As a conventional technique of restraining occurrence of high-frequency wave, in other word, harmonic current in current-feed of the heating apparatus, for example, a technique is known which turns on the current-feed by 100% when the heater temperature is less than a lower limit value, turns off the current-feed when the heater temperature is higher than an upper limit value, and a sine-wave alternating current (AC) is periodically turned on/off in synchronization with zero cross of a sine-wave AC when the heater temperature falls between the upper limit value and the lower limit value.
According to the known technique, high-frequency wave occurring at turning on/off of the sine-wave AC can be reduced. However, a standard value of a harmonic current in heaters has been recently become strict and therefore, in heating control of the heaters, a technique of further restraining the harmonic current has been demanded. The present invention provides a technique of improving the effect of restraining the harmonic current in heating control of the heaters.